Although emerging cellular system standards such as those defined by the Long-Term Evolution (LTE) initiative of the 3rd-Generation Partnership Project (3GPP) promise unprecedented data rates and flexibility for wireless systems, market pressures for ever increasing data rates are likely to continue. Of course, higher data rates generally require larger system bandwidths. For fourth-generation (4G) mobile communication systems, systems developers are discussing radio-frequency bandwidths of up to 100 MHz. However, because radio spectrum is a limited resource that must be shared between many wireless applications, wireless systems, and network operators, finding contiguous spectrum blocks to meet these needs is very difficult.
One solution to this problem is to aggregate blocks of spectrum, including non-contiguous blocks, to assemble a large system bandwidth. This can be seen in FIG. 1, where spectrum blocks 110, 120, 130, comprising two 20 MHz blocks and a single 5 MHz block, at radio carrier frequencies f1, f2, and f3, respectively, can be combined to yield a total system bandwidth of 45 MHz. With such an approach it will be possible to aggregate sufficiently large system bandwidths to support data rates up to and above 1 Gigabit per second, a throughput requirement for the 4 G, “IMT-Advanced” (International Mobile Telecommunications-Advanced) systems contemplated by the International Telecommunication Union (ITU). Furthermore, such a scenario makes it possible to adapt the aggregation of spectrum blocks to temporal and geographical constraints, making such a solution very flexible.
Several existing cellular systems, such as those defined in 3GPP's LTE specifications, may be extended in a straightforward manner to support contiguous and non-contiguous spectrum usage with the introduction of multi-carrier operation. In multi-carrier mode, two or more blocks of spectrum are aggregated, with each block corresponding to a radio-frequency carrier signal formatted and transmitted according to the existing standards for LTE. This approach is generally outlined in the 3GPP document, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Requirements for Further Advancements for E-UTRA (LTE-Advanced) (Release 8), 3GPP TR 36.913 v. 8.0.0 available at http://www.3gpp1.org/ftp/Specs/html-info/36913.htm. An LTE-Advanced mobile terminal adapted for multi-carrier operation is thus able to simultaneously receive two or more LTE carriers, each of which may have a different bandwidth, transmitted at different carrier frequencies.
A similar approach may be taken with other existing wireless standards. For instance, data targeted for a particular mobile terminal may be split between two or more distinct High-Speed Downlink Packet Access (HSDPA) carriers, at different frequencies, to achieve much higher data rates than previously possible. HSDPA-specific issues for multi-carrier operation are also currently being addressed by 3GPP, as outlined in the 3GPP document, “Technical Specification Group Radio Access Network; Dual-Cell HSDPA Operation,” 3GPP TR 25.825 V1.0.0 (2008-05), available at http://www.3gpp.org/ftp/specs/html-info/25825.htm.
As is well known to those skilled in the art, frequency and time tracking, or Automatic Frequency Control (AFC), are important aspects of mobile terminal design. Because mobile devices generally rely on inexpensive oscillators to provide a time and frequency reference, the device receiver must repeatedly estimate the time and frequency error between a received signal and the on-board reference to correctly and efficiently receive and decode transmitted data. Of course, this will also be true for multi-carrier extensions of today's HSPA and LTE system. Accordingly, improved techniques are needed for efficiently measuring and tracking frequency and time errors in a multi-carrier wireless terminal without unduly increasing the complexity of the resulting receiver design.